Of course, each person’s fasting will be unique, but here are the results of studies on humans:
Researchers fasted 45 obese people for four days in 1978 while monitoring their thyroid hormone levels before, during, and after the fast. During the fast, everyone’s levels of TSH (thyroid stimulating hormone) and T4 (inactive thyroid hormone) increased. T3 (the active thyroid hormone) levels fell for everyone. Men experienced more change than women. When they added T3 supplements, T3 remained constant. Pure carbohydrate eaters experienced a return to normal T3 levels. T3 levels returned to normal in those who consumed a variety of meals high in fat, carbohydrates, and protein.
What the people ate after the fast influenced whether their thyroid hormones returned.
Even those who simply consumed protein had reduced T3.
Even those who solely consumed fat had reduced T3.
Summary: In obese individuals without pre-existing thyroid conditions, a four-day fast reduces thyroid function. Protein or fat alone do not replenish it; refeeding with carbohydrates and/or a combination of foods does.
In 1980, scientists fasted obese human volunteers with healthy thyroid function for ten days. They consumed 1500 calories each day for five days prior to the fast. Their T3 levels decreased throughout the fast from 167 ng/dl to 86 ng/dl despite repeated TRH infusions, to which they were resistant. Typically, TRH will raise TSH.
How About Alternate-Day Fasting in hypothyroidism ?
In one study, groups of people with subclinical hypothyroidism and normal thyroid status were given a choice between alternate-day fasting (consuming 25% fewer calories on fasting days and 125% fewer calories on non-fasting days) or calorie restriction (75% fewer calories permanently) for a period of six months. About the same amount of weight and body fat were reduced by both groups. Fasting led to larger reductions in fasting insulin and insulin resistance in hypothyroid individuals.
There isn’t much published data on how fasting affects those with normal thyroid function and those who are mildly hypothyroid.
Shorter eating windows may be beneficial if you have hypothyroidism and are trying to treat it naturally.
I don’t advocate 24-hour fasts for hypothyroid patients who are on their own, but I do believe there is a case for a smaller eating window, particularly if it coincides with the day . On the other hand, if your eating window is between 9 a.m. and 5 p.m., you are eating with the sun, synchronizing your biological clock with the actual daytime.